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Subnets chosen to be used in an IP internetwork design must not overlap their address ranges. With a single subnet mask in a network, the overlaps are obvious; however, with VLSM, the overlapping subnets might not be as obvious. When multiple subnets overlap, a router’s routing table entries overlap. As a result, routing becomes unpredictable, and some hosts can be reached from only particular parts of the internetwork. In short, a design that uses overlapping subnets is considered to be an incorrect design, and should not be used.
The two types of problems that relate to overlapping VLSM subnets are analyzing an existing design to find overlaps and choosing new VLSM subnets so that you do not create an overlapped subnet.
To determine whether subnets overlap:
Step 1: Calculate the subnet number and subnet broadcast address of each subnet; this gives you the range of addresses in that subnet.
Step 2: Compare the ranges of addresses in each subnet and look for cases in which the address ranges overlap.